Field pickup units (FPUs) are known as apparatuses used in a radio communication system for video transmission such as live television broadcasting or emergency broadcasting. Such an FPU is used for transmitting materials of a broadcasting sector and configured to transmit an uplink (UL) signal of the main content information from of a scene-reporting-side mobile station (terminal) to a broadcast-station-side base station, while transmitting a downlink (DL) signal of feedback information from the broadcast-station-side base station to the scene-reporting-side mobile station (terminal). The video captured by a camera is sent via file transmission in real time and transmitted from the mobile station to the base station as an UL signal and stored in a storage medium and reproduced. In addition, a feedback signal and control signal are transmitted from the mobile station to the base station as a DL signal.
In such a radio communication system, while the DL signal which is the feedback information from a broadcast-station-side base station to a scene-reporting-side mobile station is necessary, what is desired most is faster transmission of UL signals which are the main content information such as video information used in broadcasting. An increase in the transmission rate of UL signals requires a Time-Division Duplex (TDD) frame configuration in which the UL period is longer than the DL period.
In the TDD system, a guard time is provided between a UL transmission period and a DL transmission period for the purpose of absorbing delay in radio propagation (see Patent Literature (hereinafter, referred to as “PTL”) 1. The amount of delay decreases as the distance between the mobile station and the base station decreases. Meanwhile, in order to increase the transmission rate, it is favorable to set the guard time to be as short as possible. For this reason, in the related art, the length of guard time is set to be variable so that the guard time becomes short as the distance between the mobile station and the base station decreases.
In addition, FPUs share a frequency band with another system such as a transceiver, so that the presence or absence of interference in the frequency band in use needs to be always monitored to prevent interference with the other system in FPUs. FPU base stations each measure the reception level (interference amount) of a guard time and determines that the other system has started using the frequency band (interference has occurred) when the reception level exceeds a predetermined threshold, and the base station stops using the frequency band. Note that, the reception level (reception quality) is a received signal strength indicator (RSSI), for example, to be more specific.